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PQM-702, PQM-703, PQM-710, PQM-711 Operating Manual
the Phase Locked Loop circuit as a reference signal. PLL circuit generates a frequency which is a
multiple of the reference frequency required to clock the ADC.
The need for the phase-locked loop results directly from the requirements of IEC 61000-4-7
standard, which describes the methodology and acceptable errors when measuring harmonics.
This standard requires that the measuring window (which is the basis for a single measurement
and evaluation of the harmonics) is equal to the duration of 10 mains cycles for 50 Hz systems
and 12 cycles for 60 Hz systems. In both cases, it corresponds to approx. 200 ms. Since the frequency of the mains may be subject to periodic changes and fluctuations, the duration of the window may not be exactly 200 ms, and for example for frequency 51 Hz it will be approx. 196 ms.
The standard also prescribes that before applying the Fourier formula (in order to extract the
spectral components) data should not be subject to windowing. No frequency synchronization and
a situation where FFT is performed on the samples not covering integer number of cycles, may
lead to spectral leakage. This would cause blurring of the harmonic line over a few adjacent interharmonic bands, which may lead to loss of information about the actual level and power of the
tested line. It is allowed to apply Hann weighting window, which reduces the adverse effects of
spectral leakage, but this is limited only to situations when PLL loses synchronization.
IEC 61000-4-7 specifies also the required accuracy of the synchronization block. This is expressed as follows: the time between the rising edge of the first sampling pulse and (M+1)-th
pulse (where M is the number of samples within the measuring window) should be equal to the
duration of specified number of periods in the measuring window (10 or 12) with a maximum allowable error of ± 0.03%. To explain it in a simpler way, consider the following example. Assuming
network frequency of 50 Hz, the measuring window lasts exactly 200 ms. If the first sampling
pulse occurs exactly at time t=0, then the first sampling pulse of the next measurement window
should occur at t=200 ± 0.06 ms. This ±60 µs is the permissible deviation of the sampling edge.
The standard also defines the recommended minimum frequency range at which the above-stated
accuracy of the synchronization should be maintained and defines it as ± 5% of nominal frequency, i.e. 47.5...52.5 Hz for 50 Hz and 57...63 Hz for 60 Hz .
Another issue is the input voltage range for which PLL will work properly. For this issue,
61000-4-7 standard does not provide any specific guidance or requirements. However, 61000-430 standard defines the input voltage range in which the metrological parameters cannot be compromised and for class A the range is: 10%...150% Udin. The analyzer meets the requirements
listed above relating to the operation of PLL, for the rated voltage U nom >= 100 V, i.e. up to approx.
10 V.
3.6
Frequency measurement
The signal for measuring 10-second frequency values of the network, is taken from L1 voltage
channel. This is the same signal that is used to synchronize the PLL. L1 signal is sent to a 2nd order band-pass filter, for which the passband was set at range of 40...70 Hz. This filter is designed
for reducing the level of harmonics. Then, a square signal is formed from the filtered waveform.
The signal cycles number and their duration are counted during the 10-second measuring cycle.
10-second time intervals are determined by the real time clock (every full multiple of 10-second
time). The frequency is calculated as the ratio of the number of cycles counted and their duration.
3.7
The method for measuring harmonics
Harmonics measurement is carried out according to IEC 61000-4-7.
It defines the method for calculating individual harmonics.
The whole process consists of several steps:
 synchronous sampling (10/12 periods),
 FFT (Fast Fourier Transform),
 grouping.
FFT analysis for the test window of 10/12 period (approx. 200 ms). As a result of FFT, we receive a set of spectral lines from 0 Hz (DC) to the 50-th harmonics (approx. 2.5 kHz for 50 Hz or
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